Storage element and storage device

ABSTRACT

A storage element includes a storage layer which has magnetization perpendicular to its film surface and which retains information by a magnetization state of a magnetic substance, a magnetization pinned layer having magnetization perpendicular to its film surface which is used as the basis of the information stored in the storage layer, an interlayer of a non-magnetic substance provided between the storage layer and the magnetization pinned layer, and a cap layer which is provided adjacent to the storage layer at a side opposite to the interlayer and which includes at least two oxide layers. The storage element is configured to store information by reversing the magnetization of the storage layer using spin torque magnetization reversal generated by a current passing in a laminate direction of a layer structure including the storage layer, the interlayer, and the magnetization pinned layer.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Priority PatentApplication JP 2011-114440 filed in the Japan Patent Office on May 23,2011, the entire content of which is hereby incorporated by reference.

BACKGROUND

The present disclosure relates to a storage element that has a storagelayer which stores a magnetization state of a ferromagnetic layer asinformation and a magnetization pinned layer in which the direction ofmagnetization is pinned and that changes the direction of magnetizationof the storage layer by passing a current and to a storage deviceincluding this storage element.

Concomitant with significant developments of various informationapparatuses including large capacity servers, mobile terminals, and thelike, elements, such as memories and logics, forming those apparatusesare also requested to improve performance, such as increase inintegration degree, increase in operation speed, and reduction in powerconsumption. In particular, advancement in non-volatile semiconductormemories has been remarkable, and above all, flash memories eachfunctioning as a large capacity file memory have been increasingly indemand so as to replace hard disk drives.

In addition, in consideration of expansion into code storages andworking memories, development of ferroelectric random access memories(FeRAMs), magnetic random access memories (MRAMs), phase-change randomaccess memories (PCRAMs), and the like has been pursued in order toreplace NOR flash memories, DRAMs, and the like, which are now commonlyused, and some of those memories mentioned above have been already putinto practical use.

In particular, since data is stored using the magnetization direction ofa magnetic material, the MRAM is capable of performing high-speed andalmost-infinite (10¹⁵ times or more) rewriting operations and hasalready been used in the fields of industrial automation, airplane, andthe like.

Because of its high-speed operation and high reliability, the MRAM isexpected to be expanded into the code storage and the working memory inthe future; however, in practice, there are problems to be overcome,such as reduction in power consumption and increase in capacity.

These mentioned above are intrinsic problems resulting from therecording principle of the MRAM, that is, resulting from the method inwhich magnetization reversal is performed by a current magnetic fieldgenerated from a wire. As one method to solve these problems mentionedabove, a recording method using no current magnetic field (that is, amagnetization reversal method) has been studied, and in particular,researches on spin torque magnetization reversal have been activelyperformed.

A storage element of the spin torque magnetization reversal is formedusing a magnetic tunnel junction (MTJ) as in the case of the MRAM anduses a phenomenon in which spin-polarized electrons passing through amagnetic layer pinned in a certain direction impart torque to anotherfree magnetic layer (the direction of which is not pinned) when enteringthis free magnetic layer, and the magnetization of the free magneticlayer is reversed by passing a current equivalent to or more than acertain threefold value. Rewriting of 0/1 is performed by changing thepolarity of the current.

The absolute value of the current for this reversal is 1 mA or less inan element having a scale of approximately 0.1 μm.

In addition, scaling can be performed because this current valuedecreases in proportion to the element volume. Furthermore, since a wordline for generating a current magnetic field for recording, which isnecessary for the MRAM, is not necessary in this case, the cellstructure can be advantageously simplified.

Hereinafter, the MRAM using the spin torque magnetization reversal willbe referred to as the “spin torque-magnetic random access memory(ST-MRAM)”. The spin torque magnetization reversal may also be referredto as the spin injection magnetic reversal in some cases.

The ST-MRAM has been greatly expected as a non-volatile memory that canrealize reduction in power consumption and increase in capacity whilemaintaining advantages of the MRAM, such as a high-speed operation andan almost infinite rewriting number.

As the ST-MRAM, a memory using in-plane magnetization as disclosed, forexample, in Japanese Unexamined Patent Application Publication No.2004-193595 and a memory using perpendicular magnetization as disclosed,for example, in Japanese Unexamined Patent Application Publication No.2009-81215 have been developed.

SUMMARY

Although various materials have been examined as a ferromagneticsubstance used for the ST-MRAM, in general, it has been believed that amaterial having perpendicular magnetic anisotropy is suitable forreduction in power consumption and increase in capacity as compared to amaterial having in-plane magnetic anisotropy.

The reason for this is that an energy barrier to be surpassed in thespin torque magnetization reversal is low for perpendicularmagnetization, and high magnetic anisotropy of a perpendicularmagnetization film advantageously maintains thermal stability of astorage carrier miniaturized for increase in capacity.

However, depending on a magnetic material having perpendicularanisotropy, the anisotropy energy thereof is low, and the property ofretaining information as a storage element may be a concern in somecases.

Accordingly, it is desirable to provide a ST-MRAM element which realizeshigh information-retention property as a storage element by furtherenhancing the perpendicular magnetic anisotropy and which can alsostably perform recording at a low current.

A storage element according to an embodiment of the present disclosureincludes: a storage layer which has magnetization perpendicular to itsfilm surface and which retains information by a magnetization state of amagnetic substance; a magnetization pinned layer having magnetizationperpendicular to its film surface which is used as the basis of theinformation stored in the storage layer; an interlayer of a non-magneticsubstance provided between the storage layer and the magnetizationpinned layer; and a cap layer which is provided adjacent to the storagelayer at a side opposite to the interlayer and which includes at leasttwo oxide layers. This storage element is configured to storeinformation by reversing the magnetization of the storage layer usingspin torque magnetization reversal generated by a current passing in alaminate direction of a layer structure including the storage layer, theinterlayer, and the magnetization pinned layer.

A storage device according to an embodiment of the present disclosureincludes: a storage element which retains information by a magnetizationstate of a magnetic substance; and two types of wires intersecting eachother. In this storage device, the storage element includes: a storagelayer which has magnetization perpendicular to its film surface andwhich retains information by a magnetization state of a magneticsubstance; a magnetization pinned layer having magnetizationperpendicular to its film surface which is used as the basis of theinformation stored in the storage layer; an interlayer of a non-magneticsubstance provided between the storage layer and the magnetizationpinned layer; and a cap layer which is provided adjacent to the storagelayer at a side opposite to the interlayer and which includes at leasttwo oxide layers. In addition, the storage element is configured tostore information by reversing the magnetization of the storage layerusing spin torque magnetization reversal generated by a current passingin a laminate direction of a layer structure including the storagelayer, the interlayer, and the magnetization pinned layer. Furthermore,in this storage device, the storage element is arranged between the twotypes of wires, and the current in a laminate direction passes in thestorage element through the two types of wires, so that the spin torquemagnetization reversal occurs.

In the above technique of the present disclosure, as the ST-MRAM, thestorage layer, the interlayer (tunnel barrier layer), and themagnetization pinned layer form a MTJ structure. Furthermore, the caplayer located adjacent to the storage layer includes at least two oxidelayers.

Since the cap layer is formed to have an oxide laminate structure, theperpendicular magnetic anisotropy can be enhanced as compared to that ofthe structure using a single layer oxide.

According to the technique of the present disclosure, as a non-volatilememory by a perpendicular magnetization type ST-MRAM, the perpendicularmagnetic anisotropy is enhanced, and hence high information retentionproperty (thermal stability) can be realized as a storage element.Accordingly, a ST-MRAM storage element capable of stably performingrecording at a low current and a storage device using the same can berealized.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic perspective view of the structure of a storagedevice according to an embodiment;

FIG. 2 is a cross-sectional view of the storage device according to theembodiment;

FIG. 3 is a cross-sectional view showing a layer structure of a storageelement according to an embodiment;

FIGS. 4A and 4B are views each illustrating a sample of Experiment 1according to an embodiment;

FIG. 5 is a graph showing a coercive force as the result of Experiment1;

FIGS. 6A and 6B are views each illustrating a sample of Experiment 2according to the embodiment; and

FIG. 7 is a table showing the result of Experiment 2.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described inthe following order.

<1. Structure of storage device according to embodiment><2. Outline of storage element according to embodiment><3. Concrete structure according to embodiment><4. Experiments according to embodiment>

1. STRUCTURE OF STORAGE DEVICE ACCORDING TO EMBODIMENT

First, the structure of a storage device according to an embodiment ofthe present disclosure will be described.

Schematic views of the storage device according to the embodiment areshown in FIGS. 1 and 2. FIG. 1 is a perspective view, and FIG. 2 is across-sectional view.

As shown in FIG. 1, in the storage device according to the embodiment, astorage element 3 of a ST-MRAM which can retain information by amagnetization state is arranged in the vicinity of the intersection oftwo types of address lines (such as a word line and a bit line)perpendicularly intersecting each other.

That is, a drain region 8, a source region 7, and a gate electrode 1,which form a selection transistor for selecting each storage device, areformed in an area isolated by an element isolation layer 2 of asemiconductor substrate 10, such as a silicon substrate. Among thesementioned above, the gate electrode 1 also functions as one of theaddress lines (word lines) extending in a front-back direction in thisfigure.

The drain region 8 is formed in common for the two selection transistorslocated at a right and a left side in FIG. 1, and a wire 9 is connectedto this drain region 8.

In addition, the storage element 3 having a storage layer in which thedirection of magnetization is reversed by spin torque magnetizationreversal is arranged between the source region 7 and a bit line 6provided thereabove and extending in a right-left direction in FIG. 1.This storage element 3 is formed, for example, of a magnetic tunneljunction element (MTJ element).

As shown in FIG. 2, the storage element 3 has two magnetic layers 15 and17. Of the two magnetic layers 15 and 17, one magnetic layer is used asa magnetization pinned layer 15 in which the direction of magnetizationM15 is pinned, and the other magnetic layer is used as a magnetic freelayer 17, that is, a storage layer 17, in which the direction ofmagnetization M17 is changed.

In addition, the storage element 3 is connected to the bit line 6 andthe source region 7 through respective top and bottom contact layers 4.

Accordingly, when a current is passed in the storage element 3 in atop-bottom direction through the two types of address lines 1 and 6, thedirection of the magnetization M17 of the storage layer 17 can bereversed by the spin torque magnetization reversal.

In the storage device as described above, it has been well understoodthat writing is necessarily performed at a current equivalent to or lessthan a saturation current of the selection transistor, and that thesaturation current thereof is decreased as the transistor isminiaturized. Accordingly, for miniaturization of the storage device,the current to be passed in the storage element 3 is preferablydecreased by improving the efficiency of spin transfer.

In addition, in order to increase the intensity of a read signal, it isnecessary to ensure a high rate of change in magnetoresistance, andhence, the use of the MTJ structure as described above is effective,that is, the storage element 3 in which the interlayer functioning as atunnel insulating layer (tunnel barrier layer) is provided between thetwo magnetic layers 15 and 17 is effectively used.

When the tunnel insulating layer is used as the interlayer as describedabove, in order to prevent dielectric breakdown of the tunnel insulatinglayer, the current to be passed in the storage element 3 is restricted.That is, in order to ensure the reliability for repeated writing in thestorage element 3, the current necessary for the spin torquemagnetization reversal is also preferably suppressed. In addition, thecurrent necessary for the spin torque magnetization reversal may also becalled, for example, a reverse current or a storage current in somecases.

In addition, since the storage device is a non-volatile memory device,it is necessary to stably store information written by a current. Thatis, it is necessary to ensure the stability (thermal stability) againstheat fluctuation of the magnetization of the storage layer.

If the thermal stability of the storage layer is not ensured, thedirection of reversed magnetization may be again reversed with heat(temperature in operating environment) in some cases, and a writingerror may occur.

In the storage element 3 (ST-MRAM) of this storage device, although thescaling can be advantageously performed, that is, the volume can bedecreased, as compared to a related MRAM, when the volume is decreased,the thermal stability tends to be degraded if the other properties arenot changed.

When the capacity of the ST-MRAM is increased, since the volume of thestorage element 3 is further decreased, to ensure the thermal stabilitybecomes an important subject.

Therefore, in the storage element 3 of the ST-MRAM, the thermalstability is a significantly important property, and even if the volumethereof is decreased, design has to be performed to ensure this thermalstability.

2. OUTLINE OF STORAGE ELEMENT ACCORDING TO EMBODIMENT

Next, the outline of the storage element according to an embodiment ofthe present disclosure will be described.

The storage element according to the embodiment is formed as a ST-MRAM.By the spin torque magnetization reversal, the ST-MRAM reverses thedirection of the magnetization of the storage layer of the storageelement to store information.

The storage layer is formed of a magnetic substance containing aferromagnetic layer and is configured to retain information by amagnetization state (direction of magnetization) of the magneticsubstance.

Although described later in detail, the storage element 3 according tothe embodiment has, for example, a layer structure shown in FIG. 3 andincludes at least two ferromagnetic layers, the storage layer 17 and themagnetization pinned layer 15, and an interlayer 16 providedtherebetween.

The storage layer 17 has magnetization perpendicular to its filmsurface, and the direction of the magnetization is changed correspondingto information.

The magnetization pinned layer 15 has magnetization perpendicular to itsfilm surface which is used as the basis of the information stored in thestorage layer 17.

The interlayer 16 is formed from an insulating layer, for example, of anon-magnetic substance and is provided between the storage layer 17 andthe magnetization pinned layer 15.

In addition, by injecting spin-polarized electrons in a laminatedirection of a layer structure including the storage layer 17, theinterlayer 16, and the magnetization pinned layer 15, the direction ofthe magnetization of the storage layer 17 is changed, and information isstored in the storage layer 17.

The spin torque magnetization reversal will be briefly described.

Electrons have two types of spin angular momenta. These electrons aretemporarily defined as upward and downward electrons. Inside anon-magnetic substance, the number of the upward electrons is equal tothat of the downward electrons, and inside a ferromagnetic substance,the number of the upward electrons is different from that of thedownward electrons. In the magnetization pinned layer 15 and the storagelayer 17, which are the two ferromagnetic layers forming the storageelement 3, the case in which electrons are transferred from themagnetization pinned layer 15 to the storage layer 17 will be discussedwhen the directions of the magnetic moments of the above ferromagneticlayers are in an antiparallel state.

The magnetization pinned layer 15 is a pinned magnetic layer in whichthe direction of the magnetic moment is pinned by a high coercive force.

Electrons passing through the magnetization pinned layer 15 arespin-polarized, that is, the number of upward electrons becomesdifferent from that of downward electrons. When the thickness of theinterlayer 16, which is a non-magnetic layer, is formed sufficientlysmall, before the spin polarization of electrons caused by passagethereof through the magnetization pinned layer 15 is relaxed, and theelectrons are placed in a non-polarized state (in which the number ofupward electrons is the same as that of downward electrons) in anordinary non-magnetic substance, the electrons reach the other magneticsubstance, that is, the storage layer 17.

Since the sign of the degree of spin polarization in the storage layer17 is opposite, in order to decrease energy of the system, some of theelectrons are reversed, that is, the directions of the spin angularmomenta are reversed. At this time, since the total angular momentum ofthe system should be conserved, the reaction equivalent to the total ofthe change in angular momentum of electrons, the direction of each ofwhich is changed, is also imparted to the magnetic moment of the storagelayer 17.

When the current is low, that is, when the number of electrons passingper unit time is small, since the total number of electrons, each ofwhich changes its direction, is also small, the change in angularmomentum generated in the magnetic moment of the storage layer 17 issmall; however, when the current is increased, a larger change inangular momentum can be imparted per unit time.

The change in angular momentum with time is the torque, and if thetorque exceeds a certain threshold value, the magnetic moment of thestorage layer 17 starts a precession motion and is stabilized afterrotated by 180° because of its uniaxial magnetic anisotropy. That is,reversal occurs from an antiparallel state to a parallel state.

When the magnetizations are in a parallel state, conversely, if currentis then passed in a direction such that electrons are transferred fromthe storage layer 17 to the magnetization pinned layer 15, the torque isimparted to the storage layer 17 when electrons spin-polarized byreflection at the magnetization pinned layer 15 enter the storage layer17; hence, the magnetic moment can be reversed to an antiparallel state.However, in this case, the current necessary to cause this reversal islarge as compared to that for the reversal from an antiparallel state toa parallel state.

Although it is difficult to intuitively understand the reversal of themagnetic moments from a parallel state to an antiparallel state, thismechanism may be considered in such a way that since the magnetizationpinned layer 15 is pinned, the magnetic moment is be reversed, and inorder to conserve the angular momentum of the entire system, themagnetization of the storage layer 17 is reversed. As described above,0/1 is stored by passing a current equivalent to or larger than acertain threshold value in the direction from the magnetization pinnedlayer 15 to the storage layer 17 or in the direction opposite theretocorresponding to each of the respective polarities.

Information can be read out using the magnetoresistance effect as in thecase of a related MRAM. That is, as in the case of the above storage, acurrent is passed in a direction perpendicular to the film surface. Inaddition, a phenomenon is used in which depending on whether themagnetic moment of the storage layer 17 is in the same direction as thatof the magnetic moment of the magnetization pinned layer 15 or is in adirection opposite thereto, the electric resistance of the element ischanged.

Although either a metal or an insulating material may be used for theinterlayer 16 provided between the magnetization pinned layer 15 and thestorage layer 17, it is believed that when an insulating material isused for the interlayer, a higher read signal (rate of change inresistance) is obtained, and information can be stored at a lowercurrent. The element as described above is called a ferromagnetic tunneljunction (Magnetic Tunnel Junction: MTJ).

When the direction of magnetization of a magnetic layer is reversed bythe spin torque magnetization reversal, a necessary current threshold Icis changed depending on whether the magnetization easy axis of themagnetic layer is in an in-plane direction or a direction perpendicularthereto.

Although the storage element of this embodiment is a perpendicularmagnetization type, the reverse current which reverses the direction ofmagnetization of a magnetic layer of a related in-plane magnetizationtype storage element is represented by Ic_para.

When the parallel state is reversed to the antiparallel state (theparallel state and the antiparallel state are each determined by themagnetization direction of the storage layer on the basis of themagnetization direction of the magnetization pinned layer),

Ic _(—) para=(A·α·Ms·V/g(0)/P)(Hk+2πMs) holds.

When the antiparallel state is reversed to the parallel state,

Ic _(—) para=−(A·α·Ms·V/g(π)/P)(Hk+2πMs) holds.

On the other hand, if the reverse current of a perpendicularmagnetization type storage element as that of this example isrepresented by Ic_perp, when the parallel state is reversed to theantiparallel state,

Ic _(—) perp=(A·α·Ms·V/g(0)/P)(Hk−4πMs) holds.

When the antiparallel state is reversed to the parallel state,

Ic _(—) perp=−(A·α·Ms·V/g(π)/P)(Hk−4πMs) holds.

In the above formulas, A represents a constant, a represents the dampingconstant, Ms represents the saturation magnetization, V represents theelement volume, P represents the spin polarizability, g(0) and g(π)represent coefficients corresponding to the efficiency of transmissionof spin torque to the counterpart magnetic layer in the parallel stateand the antiparallel state, respectively, and Hk represents the magneticanisotropy.

In each of the above formulas, if (Hk−4πMs) in the case of theperpendicular magnetization type is compared to (Hk+2πMs) in the case ofthe in-plane magnetization type, it is understood that the perpendicularmagnetization type is more suitable for reduction in storage current.

As a magnetic material having perpendicular anisotropy, for example, aCo—Fe—B alloy may be mentioned, and in order to realize a high rate ofchange in magnetoresistance that imparts a large read signal in theST-MRAM, the above material can be used together with MgO used as atunnel barrier (interlayer 16); hence, the combination described aboveis promising.

However, in this structure which has interface anisotropy with an oxideas the origin of the perpendicular magnetic anisotropy, a low retentionproperty (thermal stability) by lower perpendicular anisotropy energythan that of the other perpendicular magnetization materials is aconcern.

In order to improve the retention property, although there are methods,such as an increase in volume of a magnetic layer, unfavorably, thetrade-off in which the interface anisotropy is decreased as thethickness is increased is liable to occur.

When the storage element is used as a memory, information stored thereinhas to be retained.

As an index of the ability to retain information, the value of index Δ(=KV/k_(B)T) of the thermal stability is used for judgment. This A isrepresented by the following formula.

Δ=KV/k _(B) T=Ms·V·Hk·(½k _(B) ·T)

In this formula, Hk represents the effective anisotropy field, k_(B)represents the Boltzmann's constant, T represents the absolutetemperature, Ms represents the saturation magnetization amount, Vrepresents the volume of the storage layer 17, and K represents theanisotropy energy.

The influences of shape magnetic anisotropy, induced magneticanisotropy, crystal magnetic anisotropy, and the like are incorporatedin the effective anisotropy field Hk, and when the coherent rotationmodel in a single domain is assumed, this effective anisotropy field Hkbecomes equivalent to the coercive force.

3. CONCRETE STRUCTURE ACCORDING TO EMBODIMENT

Next, a concrete structure according to an embodiment of the presentdisclosure will be described.

As described above with reference to FIG. 1, in the structure of thestorage device, the storage element 3 capable of retaining informationby a magnetization state is arranged in the vicinity of the intersectionbetween the two types of address lines 1 and 6 (such as a word line anda bit line) perpendicularly intersecting each other.

In addition, when a current in a top-bottom direction is passed in thestorage element 3 through the two types of address lines 1 and 6, thedirection of the magnetization of the storage layer 17 can be reversedby the spin torque magnetization reversal.

FIG. 3 shows an example of the layer structure of the storage element 3(ST-MRAM) according to the embodiment.

The storage element 3 has an underlayer 14, the magnetization pinnedlayer 15, the interlayer 16, the storage layer 17, an oxide cap layer18, and a cap protective layer 19.

As the example shown in FIG. 3, in the storage element 3, themagnetization pinned layer 15 is provided under the storage layer 17 inwhich the direction of the magnetization M17 is reversed by the spintorque magnetization reversal.

In the ST-MRAM, the relative angle between the magnetization M17 of thestorage layer 17 and the magnetization M15 of the magnetization pinnedlayer 15 prescribes 0 or 1 information.

The storage layer 17 is formed of a ferromagnetic substance having amagnetic moment in which the direction of the magnetization is freelychanged in a direction perpendicular to its layer surface. Themagnetization pinned layer 15 is formed of a ferromagnetic substancehaving a magnetic moment in which the magnetization is pinned in adirection perpendicular to its film surface.

Information is stored by the direction of the magnetization of thestorage layer 17 having uniaxial anisotropy. Writing is performed byapplying a current in the direction perpendicular to its film surface tocause the spin torque magnetization reversal. As described above, themagnetization pinned layer 15 is provided under the storage layer inwhich the direction of the magnetization is reversed by spin injectionand is used as the basis of the stored information (magnetizationdirection) of the storage layer 17.

Between the storage layer 17 and the magnetization pinned layer 15, theinterlayer 16 functioning as a tunnel barrier layer (tunnel insulatinglayer) is provided, and a MTJ element is formed by the storage layer 17and the magnetization pinned layer 15.

In addition, the underlayer 14 is formed under the magnetization pinnedlayer 15.

The oxide cap layer 18 is formed on the storage layer 17 (that is, at aside opposite to the interlayer 16 when viewed from the storage layer17).

Furthermore, the cap protective layer 19 is formed on the oxide caplayer 18 (that is, at a side opposite to the storage layer 17 whenviewed from the oxide cap layer 18).

According to this embodiment, Co—Fe—B is used for the storage layer 17and the magnetization pinned layer 15.

In addition, besides a Co—Fe—B alloy, a Co—Fe—C alloy, a Ni—Fe—B alloy,and a Ni—Fe—C alloy may also be used for a magnetic substance formingthe storage layer 17 and the magnetization pinned layer 15.

Since the magnetization pinned layer 15 is used as the basis ofinformation, the direction of the magnetization thereof should not bechanged by recording and/or reading; however, the direction thereof isnot necessarily pinned in a specific direction, and the coercive force,the thickness, or the magnetic damping constant of the magnetizationpinned layer 15 may be increased larger than that of the storage layer17 so that the direction of the magnetization thereof is not likely tomove as compared to that of the storage layer 17.

The interlayer 16 (tunnel barrier layer) is formed, for example, of MgO.When a MgO (magnesium oxide) layer is formed, the rate of change inmagnetoresistance (MR ratio) can be increased. When the MR ratio isincreased as described above, the efficiency of spin injection isimproved, and a current density necessary to reverse the direction ofthe magnetization M17 of the storage layer 17 can be decreased.

In addition, besides magnesium oxide, the interlayer 16 may also beformed from various insulating substances, such as aluminum oxide,aluminum nitride, SiO₂, Bi₂O₃, MgF₂, CaF, SrTiO₂, AlLaO₃, and Al—N—O,dielectric substances, and semiconductors.

As the underlayer 14 and the cap protective layer 19, various types ofmetals, such as Ta, Ti, W, and Ru, and conductive nitrides, such as TiN,may be used. In addition, the underlayer 14 and the protective layer 20may be a single layer or may be formed by laminating layers of differentmaterials.

In this example, the oxide cap layer 18 has a two-layer laminatestructure including a first cap layer 18 a and a second cap layer 18 b.

The first cap layer 18 a and the second cap layer 18 b are eachpreferably formed as a layer of silicon oxide, magnesium oxide, tantalumoxide, aluminum oxide, cobalt oxide, ferrite, titanium oxide, chromiumoxide, strontium titanate, lanthanum aluminum oxide, zinc oxide, or amixture containing at least one of the oxides mentioned above.

In addition, the first cap layer 18 a adjacent to the storage layer 17is preferably a magnesium oxide layer.

Although the two-layer laminate structure of the first cap layer 18 aand the second cap layer 18 b is described in the example shown in FIG.3, a laminate structure having at least three oxide layers may also beformed.

In particular, in this embodiment, the composition of the storage layer17 is adjusted so that the intensity of an effective demagnetizingfield, which is received by the storage layer 17, is smaller than thesaturation magnetization amount Ms of the storage layer 17.

As described above, a ferromagnetic material having a Co—Fe—Bcomposition is selected for the storage layer 17, and the intensity ofthe effective demagnetizing field, which is received by the storagelayer 17, is decreased so as to be smaller than the saturationmagnetization amount Ms of the storage layer. Accordingly, themagnetization of the storage layer 17 is directed in the directionperpendicular to its film surface.

Furthermore, in this embodiment, when the insulating layer forming theinterlayer 16 is a magnesium oxide layer (MgO), the rate of change inmagnetoresistance (MR ratio) can be increased. When the MR ratio isincreased as described above, the efficiency of spin injection isimproved, and the current density necessary to reverse the direction ofthe magnetization of the storage layer 17 can be decreased.

Since the storage layer 17 of the storage element 3 is formed so thatthe intensity of the effective demagnetizing field, which is received bythe storage layer 17, is smaller than the saturation magnetizationamount Ms of the storage layer 17, the demagnetizing field received bythe storage layer is decreased, and a write current necessary to reversethe direction of the magnetization of the storage layer can bedecreased. The reason for this is that since the storage layer 17 hasperpendicular magnetic anisotropy, a reverse current of theperpendicular magnetization type ST-MRAM can be applied, and henceadvantages can be obtained in view of the demagnetizing field.

On the other hand, since the write current can be decreased withoutdecreasing the saturation magnetization amount Ms of the storage layer17, while sufficient saturation magnetization amount Ms of the storagelayer 17 is retained, the thermal stability of the storage layer 17 canbe ensured.

Furthermore, in this embodiment, the oxide cap layer 18 including thetwo oxide layers is provided in contact with the storage layer 17.

By the oxide cap layer 18 having an oxide laminate structure thusprovided, the perpendicular magnetic anisotropy can be adjusted, and thecoercive force and the information retention property (thermal stabilityindex Δ) of the storage layer 17 can be enhanced as compared to those ofthe structure using a single oxide layer.

Hence, a storage element having excellent balance in properties can beformed.

In the storage element 3 of this embodiment, the layers from theunderlayer 14 to the cap protective layer 19 are sequentially andsuccessively formed in a vacuum apparatus, so that the laminatestructure is obtained. Subsequently, the pattern of the storage element3 is formed by processing, such as etching, so that the storage element3 is manufactured.

In addition, the magnetization pinned layer 15 can be formed from asingle ferromagnetic layer or can be formed to have a laminateferri-pinned structure in which ferromagnetic layers are laminated withat least one non-magnetic layer provided therebetween.

As a material of the ferromagnetic layer which forms the magnetizationpinned layer 15 having a laminate ferri-pinned structure, Co, CoFe,CoFeB, and the like may be used. In addition, for example, Ru, Re, Ir,and Os may be used as a material of the non-magnetic layer.

Alternatively, by using antiferromagnetic coupling between anantiferromagnetic layer and a ferromagnetic layer, the structure can beformed in which the direction of the magnetization is pinned.

As a material of the antiferromagnetic layer, for example, magneticsubstances, such as a FeMn alloy, a PtMn alloy, a PtCrMn alloy, a NiMnalloy, an IrMn alloy, NiO, and Fe₂O₃ may be mentioned.

In addition, by addition of non-magnetic elements, such as Ag, Cu, Au,Al, Si, Bi, Ta, B, C, O, N, Pd, Pt, Zr, Hf, Ir, W, Mo, and Nb, to theabove magnetic substances, the magnetic properties can be adjusted,and/or other various properties, such as the crystal structure, thecrystallinity, and the stability of substances, can also be adjusted.

If the magnetization pinned layer 15 is formed to have a laminateferri-pinned structure, the magnetization pinned layer 15 can be madeinsensitive to an external magnetic field and can block the leakagemagnetic field caused by the magnetization pinned layer 15, and inaddition, the perpendicular magnetic anisotropy of the magnetizationpinned layer 15 can be enhanced by the interlayer coupling between themagnetic layers.

4. EXPERIMENTS ACCORDING TO EMBODIMENT Experiment 1

This experiment evaluated the magnetic properties of the storage element3 including the oxide cap layer 18 having an oxide laminate as shown inFIG. 3. Measurement of the magnetization curve was performed, and acoercive force Hc was measured.

Eight types of samples, Samples 1 to 8, were prepared. Samples 2 to 8are each corresponding to the storage element 3 according to theembodiment, and Sample 1 is a sample of Comparative Example.

The layer structure of each sample is shown in FIGS. 4A and 4B.

Samples 1 to 8 have the same structure as that shown FIG. 4A except forthe oxide cap layer 18.

The underlayer 14: a laminate film of a 15-nm thick Ta film and a 10-nmthick Ru film.

The magnetization pinned layer 15: a laminate film of a 2-nm thick Co—Ptlayer, a 0.7-nm thick Ru film, and a 1.2-nm thick [Co₂₀Fe₈₀]₇₀B₃₀ film.

The interlayer 16 (tunnel insulating layer): a 1-nm thick magnesiumoxide film.

The storage layer 17: a 2-nm thick [Co₂₀Fe₈₀]₇₀B₃₀ film.

The structure of the oxide cap layer 18 adjacent to the storage layer 17is shown in FIG. 4B.

Sample 1 (Comparative Example): a 0.9-nm thick magnesium oxide.

Sample 2: a 0.5-nm thick magnesium oxide and a 0.4-nm thick aluminumoxide.

Sample 3: a 0.5-nm thick magnesium oxide and a 1.0-nm thick tantalumoxide.

Sample 4: a 0.5-nm thick magnesium oxide and a 1.0-nm thick chromiumoxide.

Sample 5: a 0.4-nm thick aluminum oxide and a 0.5-nm thick magnesiumoxide.

Sample 6: a 1.0-nm thick tantalum oxide and a 0.5-nm thick magnesiumoxide.

Sample 7: a 1.0-nm thick chromium oxide and a 0.5-nm thick magnesiumoxide.

Sample 8: a 0.5-nm thick aluminum oxide and a 0.5-nm thick tantalumoxide.

In addition, on the upper part of the oxide cap layer 18 of each sample,the cap protective layer 19 (Ta, Ru, W, or the like) is laminated.

In Samples 2 to 4, magnesium oxide was used for the first cap layer 18 ain contact with the storage layer 17.

In Samples 5 to 7, magnesium oxide was used for the second cap layer 18b not in contact with the storage layer 17, and a compound other thanmagnesium oxide was used for the first cap layer 18 a.

In Sample 8, compounds other than magnesium oxide were used for thefirst cap layer 18 a and the second cap layer 18 b.

In each sample, a 300-nm thick thermally oxidized film was formed on a0.725-mm thick silicon substrate, and the storage element having theabove structure was formed the above oxidized film.

In addition, a 100-nm thick Cu film (not shown) was provided between theunderlayer and the silicon substrate.

Each layer other than the interlayer 16 was formed using a DC magnetronsputtering method. The interlayer 16 using an oxide was formed such thatafter a metal layer was formed using an RF magnetron sputtering methodor a DC magnetron sputtering method, oxidation was performed in anoxidation chamber.

After each layer forming the storage element was formed, a heattreatment was performed at 300° C. for 1 hour in an in-magnetic fieldheat treatment furnace.

(Measurement of Magnetization Curve)

The magnetization curve of the storage element of each sample wasmeasured by magnetic Kerr effect measurement.

For this measurement, instead of using an element processed bymicrofabrication, a bulk film portion having a size of approximately 8mm×8 mm specially provided on a wafer for magnetization curve evaluationwas used. In addition, a measurement magnetic field was applied in adirection perpendicular to its film surface.

FIG. 5 shows the coercive force Hc obtained from the magnetization curveof each of Samples 1 to 8.

As for the shape of the magnetization curve relating to the cap layer, amagnetization curve having a good square shape is obtained in eachstructure, and the Co—Fe—B alloy forming the storage layer 17sufficiently exhibits the perpendicular magnetic anisotropy by theinterface anisotropy.

In Samples 2 to 8 in which the cap layer has a laminate structure of twooxide layers, compared to the structure using a single oxide layer as inthe case of Comparative Example (Sample 1), the value of the coerciveforce is increased to approximately 2 times at the maximum.

The reason for this is believed that the influence of strain between thestorage layer 17 and the oxide caps 18 is reduced. As is the Co—Fe—Balloy crystallized by a heat treatment and the interlayer 16 ofmagnesium oxide, in actual materials, a local strain is introduced intothe highly oriented magnetic layer/oxide interface, and the magneticproperties are degraded.

It has been commonly understood that in particular, between a CoFe-basedmagnetic film and a magnesium oxide film, the mismatch in the latticeconstant is large, and the influence caused by strain is also large.

It is believed that in Samples 2 to 8, since an oxide having a differentmismatch is laminated in combination on a cap layer of single magnesiumoxide, the strain is modulated and compensated for, and hence themagnetic properties are improved.

In addition, in Samples 2 to 8, it is believed that since the oxideshaving different lattice constants form the laminate structure, thecompressive strain in an in-plane direction is generated in an upper ora lower portion of the laminate, and the diffusion of a Ru, a W, or a Talayer laminated to form the cap protective layer 19 caused by a heattreatment to the oxide is suppressed.

As a result, the effect of preventing degradation of the interfaceanisotropy, which occurs when the diffused cap protective layer 19 ismixed with the oxide, is obtained simultaneously together with thestrain compensation effect.

In addition, at least three oxide layers may be laminated in order toadjust the perpendicular magnetic anisotropy. In addition, besides theoxides shown in each sample, silicon oxide, cobalt oxide, ferrite,titanium oxide, strontium titanate, lanthanum aluminum oxide, zincoxide, or a mixed layer containing at least one of these oxides may alsobe contained.

When Samples 2, 3, and 4 are compared with Samples 5, 6, and 7, thestructure in which magnesium oxide is present at the interface with thestorage layer 17 has approximately 1.2 times the coercive force of thestructure in which no magnesium oxide is present at the interface.

Accordingly, magnesium oxide which can effectively exhibit interfaceanisotropy with a magnetic layer is preferably used for the first caplayer 18 a located at the interface side. However, even by an oxideadjacent to the storage layer 17 other than magnesium oxide, the effectof enhancing the coercive force can also be obtained as compared to thecase of the single oxide layer (Sample 1).

In addition, although the B composition of the Co—Fe—B alloy was set to30% in the experiment, the B composition may be changed to approximately20% to 40% in view of the TMR value and/or heat resistance.

Besides the Co—Fe—B alloy, as a magnetic material forming the storagelayer 17, a Co—Fe—C alloy, a Ni—Fe—B alloy, and a Ni—Fe—C alloy may alsobe used.

Since addition of a non-magnetic element to the Co—Fe—B alloy of thestorage layer 17 and the magnetization pinned layer 15 is alsoeffective, as the non-magnetic element in this case, Ru, Os, Re, Ir, Au,Ag, Cu, Al, Bi, Si, B, C, Cr, Ta, Pd, Pt, Zr, Hf, W, Mo, Nb, or an alloythereof may be used, and the magnetic properties can be adjustedthereby.

Experiment 2

This experiment evaluated the properties of the storage element 3including the oxide cap layer 18 having a laminate structure as shown inFIG. 3. Measurement of the magnetoresistance curve and measurement ofthe thermal stability from the reverse current value were performed.Three types of samples, Samples 9 to 11, were prepared. Samples 10 and11 each correspond to the storage element 3 according to the embodiment,and Sample 9 is a sample for comparison.

The layer structure of each sample is shown in FIGS. 6A and 6B.

Samples 9 to 11 have the same structure as that shown in FIG. 6A exceptfor the oxide cap layer 18.

The underlayer 14: a laminate film of a 15-nm thick Ta film and a 10-nmthick Ru film.

The magnetization pinned layer 15: a laminate film of a 2-nm thick Co—Ptlayer, a 0.7-nm thick Ru film, and a 1.2-nm thick [Co₂₀Fe₈₀]₇₀B₃₀ film.

The interlayer 16 (tunnel insulating layer): a 1-nm thick magnesiumoxide film.

The storage layer 17: a 2-nm thick [Co₂₀Fe₈₀]₇₀B₃₀ film.

The structure of each oxide cap layer 18 adjacent to the storage layer17 is shown in FIG. 6B.

Sample 9 (Comparative Example): a 0.9-nm thick magnesium oxide.

Sample 10: a 0.5-nm thick magnesium oxide and a 0.4-nm thick aluminumoxide.

Sample 11: a 0.5-nm thick magnesium oxide and a 1.0-nm thick chromiumoxide.

In each structure, the cap protective layer 19 formed of 5-nm thick Ruand 3-nm thick Ta is formed on an upper part of the oxide cap layer 18.

In each sample, a 300-nm thick thermally oxidized film was formed on a0.725-mm thick silicon substrate, and the above storage element wasformed on the above oxidized film. In addition, a 100-nm thick Cu film(not shown, to be formed into a word line described later) was providedbetween the underlayer and the silicon substrate.

Each layer other than the interlayer 16 was formed using a DC magnetronsputtering method. The interlayer 16 using an oxide was formed such thatafter a metal layer was formed using an RF magnetron sputtering methodor a DC magnetron sputtering method, oxidation was performed in anoxidation chamber.

After each layer forming the storage element was formed, a heattreatment was performed at 300° C. for 1 hour in an in-magnetic fieldheat treatment furnace.

Next, after a word line portion was masked by a photolithography, theword line (lower electrode) was formed by performing selective etchingby Ar plasma on the laminate film at a portion other than the word line.

In this step, the portion other than the word line portion was etched toa depth of 5 nm of the substrate. Subsequently, a mask of the pattern ofthe storage element was formed by an electron beam exposure apparatus,and selective etching was performed to the laminate film, so that thestorage element was formed.

Portions other than the storage element portion were etched right abovethe word line formed of a Cu layer.

In addition, since it is necessary to pass a sufficient current in astorage element for performance measurement in order to generate thespin torque necessary for magnetization reversal, the resistance of theinterlayer 16 (tunnel insulating layer) has to be decreased.

Hence, the pattern of the storage element was formed to have a circularshape having a short axis of 0.07 μm and a long axis of 0.07 μm so thatthe sheet resistance (Ω/μm²) of the storage element was 20Ω/μm².

Next, the portions other than the storage element were insulated byAl₂O₃ having a thickness of approximately 100 nm using sputtering.Subsequently, the bit line used as an upper electrode and pads formeasurement were formed using a photolithography.

As described above, each sample corresponding to the storage element 3was formed.

Properties of each of Samples 9 to 11 of the storage elements thusformed were evaluated as described below. In order to control thereverse current so that the value thereof in a plus direction and thatin a minus direction were symmetric to each other, before themeasurement was performed, the structure was formed so that a magneticfield could be applied to the storage element from the outside. Inaddition, the voltage to be applied to the storage element was set to 1V or less so as not to damage the insulating layer.

(Measurement of Magnetoresistance Curve (TMR Measurement))

Evaluation of the magnetoresistance curve of the storage element wasperformed by measuring the element resistance while a magnetic field wasapplied.

(Measurement of Reverse Current Value and Thermal Stability)

The reverse current value was measured in order to evaluate the writeproperties of the storage element 3 according to the embodiment. Acurrent having a pulse width of 10 microseconds to 100 milliseconds waspassed through the storage element, and the resistance of the storageelement was then measured.

Furthermore, the current to be passed in the storage element waschanged, and a current value at which the direction of the magnetizationof the storage layer of this storage element was reversed was obtained.

In addition, the dispersion of the coercive force Hc obtained bymeasuring the magnetoresistance curve of the storage element at leasttwo times corresponds to the index (Δ) of the retention property(thermal stability) of the storage element described above.

A higher Δ value is obtained as the dispersion of the coercive force Hcto be measured is smaller.

In addition, in order to take the variation between storage elementsinto consideration, after approximately 20 storage elements having thesame structure were formed, the above measurement was performed, and theaverage of the reverse current value and that of the index Δ of thermalstability were obtained.

Evaluations of the magnetoresistance curve and the magnetizationreversal properties by writing using a current of Samples 9 to 11 areshown in FIG. 7.

The TMR (tunnel magnetoresistance effect) value, the coercive force Hc,the thermal stability index Δ, and the reverse current density JcO areshown.

In Samples 10 and 11, it is found that as in the case of the comparisonusing the bulk film performed in Experiment 1, while the TMR (tunnelmagnetoresistance effect) value and the reverse current density aremaintained similar to those of Comparative Example (Sample 9), thecoercive force Hc and the retention property (thermal stability index Δ)are increased to 1.1 to 1.4 times.

From these results, the advantage of the oxide cap layer 18 having alaminate structure was confirmed.

In addition, besides the structures of Samples 10 and 11, the laminatestructure of the oxide cap layer may be changed within an effectiverange as described in Experiment 1.

In addition, by changing Ru of the cap protective layer 19 in contactwith the oxide cap layer 18 to another material, the structure may beformed so as to decrease the reversal current.

As apparent from Experiments 1 and 2 described above, the storageelement 3 according to this embodiment has an effect of easilymanufacturing a perpendicular magnetization type MTJ, a large capacityand a low power consumption ST-MRAM storage element using the above MTJ,and a storage device using this storage element.

This technique can also take the following structure.

(1) There may be provided a storage element including a storage layerwhich has magnetization perpendicular to its film surface and whichretains information by a magnetization state of a magnetic substance; amagnetization pinned layer having magnetization perpendicular to itsfilm surface which is used as the basis of the information stored in thestorage layer; an interlayer of a non-magnetic substance providedbetween the storage layer and the magnetization pinned layer; and a caplayer which is provided adjacent to the storage layer at a side oppositeto the interlayer and which includes at least two oxide layers, and inthis storage element, information is stored by reversing themagnetization of the storage layer using spin torque magnetizationreversal generated by a current passing in a laminate direction of alayer structure including the storage layer, the interlayer, and themagnetization pinned layer.(2) In the storage element of the above (1), the cap layer includes alayer of silicon oxide, magnesium oxide, tantalum oxide, aluminum oxide,cobalt oxide, ferrite, titanium oxide, chromium oxide, strontiumtitanate, lanthanum aluminum oxide, zinc oxide, or a mixture containingat least one of these oxides mentioned above.(3) In the storage element of the above (1) or (2), of the at least twolayers forming the cap layer, a layer adjacent to the storage layer is alayer of magnesium oxide.(4) The storage element of one of the above (1) to (3), a ferromagneticlayer material forming the storage layer is Co—Fe—B.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. A storage element comprising: a storage layer which has magnetizationperpendicular to its film surface and which retains information by amagnetization state of a magnetic substance; a magnetization pinnedlayer having magnetization perpendicular to its film surface which isused as the basis of the information stored in the storage layer; aninterlayer of a non-magnetic substance provided between the storagelayer and the magnetization pinned layer; and a cap layer which isprovided adjacent to the storage layer at a side opposite to theinterlayer and which includes at least two oxide layers, wherein thestorage element is configured to store information by reversing themagnetization of the storage layer using spin torque magnetizationreversal generated by a current passing in a laminate direction of alayer structure including the storage layer, the interlayer, and themagnetization pinned layer.
 2. The storage element according to claim 1,wherein the cap layer includes a layer of silicon oxide, magnesiumoxide, tantalum oxide, aluminum oxide, cobalt oxide, ferrite, titaniumoxide, chromium oxide, strontium titanate, lanthanum aluminum oxide,zinc oxide, or a mixture containing at least one of these oxides.
 3. Thestorage element according to claim 1, wherein in the cap layer includingat least two oxide layers, an oxide layer adjacent to the storage layerincludes magnesium oxide.
 4. The storage element according to claim 1,wherein the storage layer includes a ferromagnetic layer materialcontaining Co—Fe—B.
 5. A storage device comprising: a storage elementwhich retains information by a magnetization state of a magneticsubstance; and two types of wires intersecting each other, the storageelement including a storage layer which has magnetization perpendicularto its film surface and which retains information by a magnetizationstate of a magnetic substance; a magnetization pinned layer havingmagnetization perpendicular to its film surface which is used as thebasis of the information stored in the storage layer; an interlayer of anon-magnetic substance provided between the storage layer and themagnetization pinned layer; and a cap layer which is provided adjacentto the storage layer at a side opposite to the interlayer and whichincludes at least two oxide layers, wherein the storage element isconfigured to store information by reversing the magnetization of thestorage layer using spin torque magnetization reversal generated by acurrent passing in a laminate direction of a layer structure includingthe storage layer, the interlayer, and the magnetization pinned layerand is arranged between the two types of wires, and the current in alaminate direction passes in the storage element through the two typesof wires, so that the spin torque magnetization reversal occurs.